Denver’s Autumn and Winter Outlook

It’s no secret that the hot and sweaty dog days of summer are over. There’s fresh snow on the mountain tops, grey skies, and even a few changing leaves around town. Over the last few days we’ve noticed a big dip in temperatures around the Rocky Mountain region and we’re beginning to enter a period with seasonally average temperatures, which is a nice departure from what has become the hottest year on record across the globe. However, we seem to have returned to 90° temperatures after having similar cool downs in the last two months. So, there are some real questions

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I70 Eastbound Near Frisco on September 16th, 2016

as far as weather cooler temperatures will continue and, realistically, what Fall and Winter will have in store for us.

Let’s dive into these questions, but first, we’ll examine how seasonal forecasts are made, their strengths and shortcomings. As always, I’m going to break this into sections. If you want to skip the science-y stuff and go right to the forecast! No problem, just skip ahead.

Sections

  1. Climate and Weather Models
  2. The Farmer’s Almanac
  3. El Nino/La Nina Impacts on Colorado Weather
  4. Fall and Winter Forecasts

 

Before we continue, something important to note is that climate and weather are NOT the same thing. Weather is short term, smaller in scale and much more erratic. Climate is long term, larger in scale (think all of Earth) and follows trends.

Climate and Weather Models

Trade offs, trade offs, trade offs! It’s no secret that we are not very good at predicting the weather far in advance, but few people actually know why it’s so difficult. It’s comes down to trade offs between length of prediction (how far into the future we’re forecasting) and the size of the area we’re trying to predict. Why? Why does this trade off exist? We’re going to start with something that I saw from my professor, Dr. Sam Ng, multiple times when I was in school. Get out the bongos and get ready to snap your fingers because it’s a poem! Yes, that’s right, a poem by Lewis Fry Richardson who actually changed it from someone else’s poem, but that doesn’t matter.

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WRF Model of Hurricane Sandy

“Big whirls have little whirls,
That feed on their velocity;
And little whirls have lesser whirls,
And so on to viscosity.”

Ok, I know what you’re thinking, “This poem sucks”, and you’re not wrong, it does. However, it perfectly describes our current issue with weather and climate modeling. When we talk about the atmosphere we see interactions everywhere. We see the transport of temperature, moisture, soil, gases, mass, aerosols, etc. These motions don’t exist only at large planetary scales, they shrink down to be infinitely small. They also exist in three dimensions that can interact with each other! That’s a “duh!” statement, but we have to make sure that we’re on the same page. Each parameter of the environment has its own unique set of governing equations in addition to the overall equations that describe the motion and evolution of the atmosphere and current computational power isn’t enough to get long range forecasts on small scales that can predict snowfall on a neighborhood by neighborhood basis. Hence, why there are so many botched forecasts.

For example, the NCAR supercomputers that we use to run the Weather Research and Forecasting (WRF pronounced warff) model can compute 5.34 quadrillion calculations per second and yet, they are still extremely limited in their processing abilities for detailed models.

It all comes down to grid spacing within the model structure. Our most detailed operational model, the High Resolution Rapid Refresh (HRRR), operates at 3km grid spacing and forecasts out to 24 hours in the future. It is generally assumed that, in order to resolve a weather feature, three grid points are needed. This means that our most detailed model can only resolve features that are bigger than approximately 5.5 miles. Now 5.5 miles sounds small,  but it could mean the difference between the model predicting that downtown Denver will get hit and then only the areas west of Wadsworth seeing snow and Denver not getting any. Sound familiar from last year when the foothills and southern areas got snow and Denver saw almost nothing?

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HRRR Model Forecast Vs. Observed Storm System- Good but not perfect

Due to its high resolution, the HRRR can only realistically be run for short forecasts in the future before it becomes computationally expensive/problematic. It is very reliable for short term forecasts and detail in small areas, but it comes at the cost of only being short term. By comparison, our climate models do very well with long range forecasting, but at the price of detail. We can accurately predict future climate patterns, but we won’t be able to have the fine details of individual weather systems and rainfall/snowfall. So, there’s that trade-off mentioned earlier. We can either use all our processing power on looking far into the future or we have to focus it on looking at very small areas. We can’t do both.

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Now, this is assuming that all the data that we have pumped into these models is absolutely perfect and, you know what? It’s just not. We do a fairly good job of making measurements of the atmosphere and feeding it into the models, but the instrumentation sites are sparse and have limitations and that can create problems. Imagine an errant temperature measurement propagating forward in space and time every five minutes for thirty days! SCARY! Overall, our long range (climate) models and very short range (weather) models perform well. It’s those intermediate models that tend to have issues. So, when we are asked what the future holds and what the models are saying about seasonal weather in a particular region, it’s not as straight forward as it sounds.

The Farmer’s Almanac

For some reason, people LOVE the forecasts presented by the Farmer’s Almanac. I’m not entirely sure whether it’s due to nostalgia, good marketing, or maybe people just like to say “almanac”. It is kind of fun to say. Sounds a bit like a french pastry. “I’ll have two croissants and a raspberry almanac!” Anyway, the Farmer’s Almanac really isn’t any more reliable that any other forecast out there (maybe AccuWeather’s 90 day forecast) and its approach to forecasting weather is fairly common sense. They don’t havefarmers-almanac_0 a gypsy fortune teller tied to a weather tower or anything.

According to their webpage-

“We derive our weather forecasts from a secret formula that was devised by the founder of this Almanac, Robert B. Thomas, in 1792. Thomas believed that weather on Earth was influenced by sunspots, which are magnetic storms on the surface of the Sun.”

Apart from the rest of their “secret formula”, which I’m fairly sure is just a fancy way of saying “climatology”, sunspots can be an indicator of weather patterns. EVERYTHING in our atmosphere is driven by solar radiation. Without it, we’re not evaporating water for clouds, temperatures would remain constant and fronts wouldn’t exist (neither would high and low pressure centers), nothing would be happening. So, when a sunspot appears and releases increased amounts of solar radiation, chances are high that we’re going to see additional warming. This is a gross oversimplification of the processes involved, but it conveys the point.

Now, there’s another possible explanation for the love of The Farmer’s Almanac and that’s what’s known as confirmation bias. Many people swear that it’s the only accurate source for weather and, after hearing that enough, others start to believe it and will look for ways (intentionally and unintentionally) to show that it is right. So, when it nails a forecast, everyone praises the Farmer’s Almanac like they did with the “Polar Vortex” outbreaks. However, when Boston had its record breaking snowfall in the 2014/2015 winter, the Almanac had failed to predict it and no one paid attention to it. A prime example of confirmation bias where you only search for data that supports what you already believe. Realistically, it is a good source of rough estimates, but it’s no better than any other source at predicting weather.

As a side note, the Polar Vortex really doesn’t travel into America. Al Roker is an idiot. The Polar Vortex is named for the region that it circles which is…. the north pole. What Al Roker was referring to was an Arctic Cold Front which brings very cold arctic air southward.

El Nino/La Nina Impacts on Colorado Weather

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Higher than normal sea-surface temperatures (yellow and red) west of South America during the 1997 El Nino

This is something that needs to be touched on from time to time to remind, even myself, to be aware that there are problems with attributing Denver’s weather to ENSO (El Nino Southern Oscillation). Briefly, increased sea surface temperatures that extend westward for northern South America are classified as El Nino years. Conversely, La Nina years are times with cooler sea surface temperatures. Most often we hear people attribute big snow storms to one or the other.

“This 34 inch blizzard is awesome! I knew El Nino would come through!”

However, looking at the impacts of El Nino or La Nina on a single weather event in Denver isn’t realistic. In fact, there is almost a perfect 50/50 split of Denver’s largest blizzards between El Nino and La Nina years. That’s not to say that it doesn’t effect the weather. It effects weather patterns in a big way and can bring differences in temperature and moisture to different areas, but as far as Denver is concerned, there has never really been a correlation drawn between the ENSO cycle and our weather. This means that all those “We’re transitioning into a La Nina year so we can expect _______!” statements simply aren’t true.

Fall and Winter Forecasts

Over the last week, we’ve seen an extended period of 60 and 70 degree temperatures around the Denver metro area. While a resurgence of mid-80s temperatures will occur this weekend and into the beginning of next week, it looks like our 90 degree days are behind us as we will again see 70s late next week. The current models are showing a typical cool down over the next few weeks and seasonal high temperatures, but they’re also showing almost no precipitation at all. Unfortunately, that looks like a trend that will continue all winter.

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Probabilities of higher or lower temperatures from October to December. Orange and reds are above average, blues are below average temperatures.

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Probabilites of increased or decreased precipitation. Greens are above average, yellows are below average.

Right now, Denver is looking at a warmer than average winter according to the latest model runs and, unfortunately, that includes the Rocky Mountains as well. The figures above show increased temperature across most of the USA and an equal chance (EC) for above or below normal precipitation. One area of concern is that even if we get plenty of snowfall in the mountains, the warmer temperatures will ensure that it melts off faster and that our water supply is lower come summer. If the current observable trends are any indication, we could be entering into another drought stage later this winter and spring but only time will tell. So, from a climate/seasonal perspective, more of the same. Record breaking heat and drought over much of the United States.

As far as Denver is concerned, it looks like we’ll be warmer than average through all of winter based off of the latest models and information. How much snow or rain we’ll get has yet to be seen, but a wet winter isn’t looking particularly promising.

As always, if you have any questions, feel free to ask here or on Facebook and I’ll get back to you!

Thanks for reading!

-Andrew

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Denver’s July Weather and June Summary

June Summary

June finished up with a total of 1.62 inches of rain, which is .36 inches below normal. The average temperature was 70.8 degrees F, which is 3.4 degrees above normal. I find the precipitation number to be a bit misleading since the official Denver station is at DIA. The airport only recorded a trace of rain while downtown got pounded by a strong thunderstorm on the 28th. Finishing up the month 3.4 degrees above average is pretty significant, but not enough to get in the top 5 warmest June’s. 12 days were above 90 degrees with 98 as the hottest temperature recorded.

July Climatology

July is Denver’s hottest month on average with normal highs in the upper 80’s and low 90’s, and average low’s around 60. July tallies 2.16 inches of rain on average, almost a full inch less than June! (With .61 inches of rain already recorded on the 1st of this month, we’re already 1/4th the way there!) The typical day consists of a warm sunny morning, and hot early afternoons followed by afternoon clouds and showers, more so towards the end of the month when the North American Monsoon kicks in. The North American Monsoon is a seasonal change in the upper level patterns. The subtropical ridge, which has clockwise flow around it, is usually centered just to our east or south brining moist air all the way from the Gulf of Mexico. This is moisture in the upper levels, which usually translates to high-based thunderstorms that don’t provide a whole lot of rain, and can be wildfire starters. Cool fronts from the north can slip in cooling things off about 10 degrees and helping to spark some storms.

July 2016 Outlook

Looking at patterns that could affect our monthly weather, one major player is the ENSO oscillation (El Nino and La Nina) over the tropical Pacific Ocean. Summer effects are less so than other seasons, but there is still some correlation. In years transitioning from an El Nino to a La Nina as we likely are this year, July was slightly warmer and wetter than average (Figures 1, 2). The average upper level pattern during those years had a ridge centered just to our south, bringing the usual monsoonal moisture our way (Figure 3). There are other oscillations that can alter long-term weather patterns such as the Pacific North American Oscillation, but those patterns don’t have a whole lot of correlation to our weather in July. Current CFS model runs show a general trough to our west and in our area during the second week of the month indicating normal to slightly cooler temperatures. The subtropical high is shown to build back in during the second half of the month leading to warm temperatures and some monsoonal flow. The Climate Prediction Center is calling for a slightly warm and dry first half of the month, and a warm second half with near average precip.

Enjoy the weather!

JulyTempAnomLaNina

(Figure 1) Temperature Anomalies in Celsius, warm colors are above average temperatures, cool colors below normal.

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(Figure 2) Precipitation anomalies in mm per day. Blue colors indicate above average precip.

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(Figure 3) Average 500mb Heights with clockwise flow around the High pressure center

Severe Storms for Denver Today with Large Hail and Possibly a Weak Tornado or Two

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UPDATE — THE NATIONAL WEATHER SERVICE HAS ISSUED A TORNADO WATCH FOR THE DENVER METRO AREA AND MOST OF THE EASTERN PLAINS UNTIL 9PM. THIS MEANS THAT CONDITIONS ARE RIGHT FOR TORNADOES TO FORM. tornadowatch
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We’re in the heart of Colorado severe weather season and leading up to this point, we’ve been relatively quiet. The occasional severe storm with hail or strong wind has swiped across the metro area before heading out east, but our severe weather seems to be a lot quieter than at this same time last year. Today, however, that looks to change. Denver and the surrounding areas have an Enhanced risk of thunderstorms today as put out by the Storm Prediction Center. This is something that generally only happens once or twice per year in Denver and is generally a good indication that we are in for some intense weather.

Enhanced

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Timing and Risks

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A line of storms has already developed to the south and west of Denver along the foothills and looks to be growing to the north. We’re going to start seeing storms develop around Denver and they’ll really pick up in intensity around noon.

Currently, it’s looking like the most significant storms will come through the area around noon and will be followed by a secondary push around 4pm after which storms will continue through tonight. The area at highest risk for damage from the storms is southeastern of Denver in areas like Highlands Ranch, Parker, Centennial, and extreme southeastern Aurora.

The atmosphere has plenty of moisture at the mid to upper levels which means that our most significant threat will be large hail. The latest models show that we lack a lot of the lower level shear (rotation from wind) to support intense tornadoes, however, we will have a southeasterly component to our wind which means that the Denver Convergence Vorticity Zone (DCVZ), which is an area where winds of opposing directions create rotation, will exist to the east of Denver. This area that extends northward along the I-25 corridor from Castle Rock to Denver and eastward from Denver to Limon is notorious for spawning weak landspout tornadoes and should be watched closely this afternoon. With that said, hail will be the primary threat followed by possibly strong and damaging winds. The NWS has not issued a tornado watch at this time. Tornadoes should be limited and, based off of current models, will be weak if they develop at all.

Chance of Thunderstorms, then Heat!

Thunderstorms are expected to develop in the evening hours on Monday, with a better chance of rain on the South and West sides of town. There is also a chance of storms in the afternoons on Tuesday and Wednesday. Some storms could be strong with high winds and decent sized hail on Tuesday if they are able to get going. Temperatures should be in the low to mid 80’s for highs each day. The weather will turn hot with temperatures around 90 from Thursday through the weekend. An approaching system may cool us down a bit next Monday. There will be a small chance for storms each day, mainly in the mountains and on the Palmer Divide.

Enjoy the weather and don’t forget the sunscreen!

Denver’s June Weather

June is typically warm and pleasant in the Denver area, with average highs ranging from the upper 70’s at the start of the month to the mid 80’s at the end of the month. Rainfall usually comes in the form of thunderstorms, some of which can be severe as June is part of Denver’s severe weather season. Cold fronts coming from the north bring cooler temperatures, but can also enhance the severe weather threat a day or two later, as moisture sometimes is pushed into the area from easterly winds. June can also feature very hot temperatures, as was the case in 2012 when Denver tied its all-time record high of 105º F, two days in a row on the 25th and 26th in a 5 day heat wave featuring highs above 100 every day.

The intense El Nino event that occurred over the last year has weakened rapidly and will likely transition to a La Nina in the fall and winter. The ENSO cycle as it’s called, can lead to changes in the upper level weather patterns across the globe, including the US. Comparing four previous years with a transition from El Nino to La Nina conditions in the summer (1988, 1998, 2007, 2010). Warmer than normal temperatures occurred in the center of the country, with Denver averaging about 1.4º F warmer than average on those years (Figures 1 and 2). Rainfall was also less than normal during these transition years, averaging about 0.7 inches less (Figure 3). Denver’s average June rainfall is just under 2 inches, which translates to about a third less rainfall than normal during the transition years.

JuneSfcTempMean

Figure 1 Average Air Temperature for June in Celsius. (16C = 61F)

JuneSfcTempAnom

Figure 2 Difference from Average Temperature During Transition Years (Warm colors are hotter, cool colors are colder)

JunePrecipAnom

Figure 3 Difference from Average Rainfall During Transition Years (Warm colors show less rain, cool colors are more)

2016 Outlook

(A brief explanation of what upper level ridges and troughs mean: Ridges are usually associated with dry and warm weather, while troughs are associated with cooler and more unsettled weather.)

Looking ahead to this June, weather models show a ridge in the jetstream to the west and near our area for about the first 10 days, indicating generally dry and warm conditions (Figure 4). That could change around the 8th, when we could see a weather system bring us a few thunderstorms and cooler temperatures. A stronger system is predicted to move into our area around the 10th, bringing cooler temperatures and the chance for more precipitation. Beyond this, the GFS (a mid range weather model) has a general ridge centered over us or just to our west mid-month, while the CFS (a long range climate model) has a ridge centered to our east. Both models have warm temperatures either way.

RidgeTrough

Figure 4 Jetstream on June 5th

Looking farther out gets tricky, as models rarely make the correct forecast. However, the CFS does a pretty good job at predicting general trends, and it has been showing an upper level ridge centered to our east and a trough to our west for most of the remainder of June. If this verifies, we could see generally warmer conditions to finish the month. (Sounds a lot like the pattern in the previous transition years doesn’t it?)

The Climate Prediction Center is calling for a warmer first half of June, with about average precip. The rest of the month is shown to have equal chances of above or below temperatures and precipitation. Only time will tell!

 

Enjoy the weather! I will likely be posting monthly outlooks and other interesting events in the future.

Jon

 

For more info about these topics check out:

(El Nino Discussion) http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

(Climate Re-analyzer) http://www.esrl.noaa.gov/psd/

(Forecasting) http://www.weather.cod.edu/forecast/

 

 

Severe Thunderstorms Today for Denver and Parts of Northeastern Colorado

 

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Tornado near Cheyenne Wells, Colorado May 9th, 2015

Well, we’ve arrived in severe weather season and, while it has been roaring in the central high plains for a while, it’s starting to pick up in Colorado now. We’ve already seen a few tornadoes in extreme eastern Colorado and now it looks like the hail, wind, and tornado threats are starting to move to the west and will be in the front range before too long. While June is typically the busiest month for severe thunderstorms and tornadoes in Colorado, May has its fair share of wild weather events and serves as an introduction for the season.

Today we’ll start our severe weather season for the Denver metro area with a small chance of some hail and high winds although any significant hail and any tornadoes should stay to the east of town along I-76. High moisture in the region will feed storms that currently look like they’ll start on the eastern side of the metro area. On days when we see southeasterly winds (from the southeast), a feature called the “Denver Cyclone” tends to develop which is a broad circulation of air caused by our unique terrain that spins counter-clockwise. The Denver Cyclone aids in the development of thunderstorms and can develop areas of colliding air that produces tornadoes around Aurora, Parker, and Elizabeth. Today, however, any tornado threat should be along I-76.

Understanding Thunderstorm Threats

Below is the Storm Prediction Center’s storm outlook for today. The different categorical outlooks are familiar for those that have lived in places like Texas, Oklahoma, and Kansas. However, those of us that have lived in Colorado or in other locations may not be as familiar with the classification system. The Denver area and the northeastern Colorado plains is under a slight risk area which means that there could be hail between 1-2″, damaging winds, and one to two tornadoes. The I-76 corridor is under an enhanced risk for severe storms which means that severe storms will be fairly widespread, damaging hail and wind reports will be common, and a few tornadoes are possible. A graphic describing severe thunderstorm threats has been added below. The threat category colors correspond to the colors on the map.

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SPC’s Severe Weather Outlook for May 24, 2016

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Forecast

Storms will develop to the east and north of the Denver area early this afternoon between 1-2 pm local time. These storms will intensify and will follow the I-76 corridor to the northeast and will eventually turn into a line of storms that exits the state into Nebraska and Kansas. A primary storm will have the possibility to produce large hail, strong damaging winds, and a few tornadoes. Towns in the immediate impact area include Akron, Brighton, Brush Lochbuie, Fort Morgan, Holyoke, Sterling, Wiggins, Wray, Yuma.

Updates will be made during the day as storms occur. Ask any questions that you have.

 

-Andrew

Cold And Soggy Weekend In The Forecast for Denver

We have had plenty of moisture flow into Colorado over the last two months and it has helped us increase our water tables and keep Colorado out of drought that tends to take hold when we have drier springs. While southeast Colorado is still a bit drier than normal, there isn’t a single location in Colorado designated as being in a drought which is great news to a state that tends to be ravaged by wildfires when droughts take hold. current_usdmIt looks like today and this weekend will help to keep our risk of drought low as we have plenty of cool temperatures and water headed into the state from today through Tuesday. While there is some disagreement on how much moisture we will see, there is agreement that it will be a significant amount and that it will continue for several days.

Current Conditions and Forecast

Snow and rain is beginning to develop around the metro area and is moving into the area from the south as the storm system entering the state begins to spill over the mountains. In the image below, you can see a satellite image of atmospheric water vapor from the GOES West satellite that shows a concentration of moisture from the Pacific Ocean just to the west of Colorado that has started to impact the western slope. Over the next several hours, we will see that concentration of moisture move over the mountains and into the Denver metro area which will increase rain coverage during the day and will lead to occasionally heavy rainfall this afternoon and evening.

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GOES West Satellite Water Vapor Imagery April 28, 2016 8:34 am

Tonight a low pressure center will position itself over southeast Colorado that will aid in bringing moisture to the region through upslope wind flow and our moist beginning to the weekend will continue. Our rain will switch to snow tonight after midnight and should continue as snow through most of tomorrow due to high temperatures only being in the mid 30s. Currently, it looks like most regions will get 1-3″ of snowfall with higher amounts at higher elevations. However, due to our recent warm temperatures, most of the snow will melt off and very little accumulation will actually take place on the ground other than on grassy surfaces.

A fair amount of agreement exists between the models with the total amount of liquid water out of this storm system. One model is saying that we’re looking at roughly 1.5″ of total liquid water and the other is saying roughly 2″. So, there are still some unknowns with this system, but we’re certain that there will be a lot of precipitation. I should mention that the models are suggesting that this will be more of a snow event than I’m predicting, but due to the temperatures and the bulk of the moisture coming when we’re warm, I think they’re erroneous in their prediction.

Final Thoughts

As of right now, it looks like we’ll get the majority of our moisture today and tomorrow, but we’ll continue with cold and soggy conditions through Tuesday with rain falling during the day and snow (or a rain/snow mix) at night. Our temperatures will fall to a minimum tomorrow when highs will only be in the mid 30s. From there, we’ll slowly climb into the 40s on Saturday and Sunday and possibly the low 50s on Monday.

So, prepare for a cold and soggy weekend that will eventually lead to sunny skies, but not before the middle of next week. This type of extended moist and cloudy system is not something that we’re used to dealing with here, but if Seattle can handle it all year long, we can make the best of it for a few days!

 

Thank you for reading!

-Andrew

 

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